It is well known that an induced current can be generated in a semiconductor having a p-n junction or Schottky Barrier (metal-semiconductor rectifying contact) by shining a focused beam of radiation of above bandgap energy on the body of the semiconductor. Apparatus for generating such beams and for scanning them across a device under test (DUT) are commercially available. In the case of a large area device, such as a high-voltage HV LDMOS transistor (which typically is ten or more microns in length) it is convenient to use a laser beam focused through an optical microscope to illuminate and scan the device. It is to be understood however that other radiant energy beams, such as an electron beam (EBIC), can be used for this purpose and are the method of choice for small electronic devices.
High voltage HV LDMOS devices, particularly those fabricated utilizing the reduced surface electric field (RESURF) principle, generally require very specific (e.g., linearly increasing) doping profiles in the drift region of the device. A voltage-capacitance relationship internally of a LDMOS transistor determines a space charge region depletion width "W" in the transistor as a function of an applied reverse bias voltage as the transistor is scanned by a laser (or other radiant energy) beam. This reverse bias voltage determines how many trapped positive (donor) ions can be uncovered in the graded-dopant n-type drift region of the transistor and results in a fixed depletion width W for each applied voltage. As will be explained in greater detail hereinafter a beam-induced photocurrent signal from the LDMOS device under reverse bias allows the measurement of the depletion width W as a function of applied reverse bias voltage.
It is desirable to be able to make quick, accurate and non-destructive measurement of dopant impurity concentrations and profiles along the length of certain semiconductor devices such as LDMOS transistors. This provides for the rapid monitoring of the design and functionality, and the manufacturing processes for such devices. Prior to the invention, so far as is known, no one had utilized a radiant beam scanning system for such measurements.